net: dsa: mv88e6xxx: Abstract HW timestamp setup

The 6165 family does not have per port PTP control registers. Also, it
places the timestamp data in different registers. Abstract the current
implementation of 6352 compatible PTP devices so that 6165 can be
added.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>